Photosynthesis is a process used by plants and other organisms (i.e., protists and cyanobacteria) to convert light energy into chemical energy that can be later released to fuel the organisms' activities. This chemical energy is stored in carbohydrate molecules, such as sugars, which are synthesized from carbon dioxide and water. In most cases, oxygen is also released as a waste product. Most plants, most algae, and cyanobacteria perform photosynthesis; such organisms are called photoautotrophs. Although photosynthesis is performed differently by different species, the process always begins when energy from light is absorbed by proteins called reaction centres that contain green chlorophyll pigments. In plants, these proteins are held inside organelles called chloroplasts, which are most abundant in leaf cells, while in bacteria they are embedded in the plasma membrane. In these light-dependent reactions, some energy is used to strip electrons from suitable substances, such as water, producing oxygen gas while fixing carbon molecules from CO2. Furthermore, two further compounds are generated: reduced nicotinamide adenine dinucleotide phosphate (NADPH) and adenosine triphosphate (ATP), which provide energy to cells.
In plants, algae and cyanobacteria, sugars are produced by a subsequent sequence of light-independent reactions called the Calvin cycle, but some bacteria use different mechanisms, such as the reverse Krebs cycle. In the Calvin cycle, atmospheric carbon dioxide is incorporated into already existing organic carbon compounds, such as ribulose bisphosphate. Using the ATP and NADPH produced by the light-dependent reactions, the resulting compounds are then reduced and removed to form further carbohydrates, such as glucose.
Photoreactors, in particular photobioreactors, are known, and have been used for the industrial production of microalgae, for example Chlorella sp, Chlamydomonas sp or Haematococcus pluvialis, photosynthetic bacteria such as for example cyanobacteria (e.g., Arthrospira, Rhodobacter, Rhodospirillum), mosses or other plant cell cultures.
Open channel photobioreactors, such as ponds, experience difficulties from contamination by other photosynthetic organisms, parasites, predators or external pollutants and from the inefficient use of light that illuminates only the top portion of the pond. More efficient photobioreactors will have an illumination surface area per unit volume (S/V) ratio that is high, shallow ponds are the norm. This, however, greatly increases land space requirements for pond-based photobioreactors. In addition, when these ponds use natural sunlight the process is limited by the available hours of sunlight. Such processing limitations can be important if the photobioreactors are used to process waste gasses from polluting facilities that operate twenty-four hours a day. Further, if these ponds are not insulated from the elements such as seasonal changes in weather, the photosynthetic organisms will find it difficult to withstand changes in temperature, external pollution, and attack from hostile species. Other limitation found in open ponds and raceways is the inefficient mixing, that leads to low productivities due to low mass transfer of gasses such as CO2 that limits the growth or oxygen that, inhibits the growth. Lack of control and laminar flows end in sedimentation of the cultures and biofouling, decreasing productivity and increasing maintenance and production costs.
An alternative technology that has received considerable attention is the closed channel system such as those systems having cylindrical tubes that employ the airlift principle. In general, airlift photobioreactors have photosynthetic material such as algae suspended in a liquid medium into which air or gas is injected into the bottom of the system, which then rises through the fluid medium in the cylindrical tube. Conventional airlift photobioreactors, however, suffer from the lack of easily definable flow patterns that can be duplicated or controlled. It is further known to provide an apparatus for culturing photosynthetic microorganisms having at least one bioreactor and at least one source of electromagnetic radiation wherein the at least one source of electromagnetic radiation is a light emitting diode.
One of the key industry drivers for the development of this technology has been the need to produce biofuels as a substitute for fossil fuels, as clearly stated by Germany's Fraunhofer Institute for Interfacial Engineering and Biotechnology “currently biofuels are mainly produced from plant-based raw materials for example biodiesel from rapeseed or palm oil. In Germany the arable land will no longer be available for food production; in Southeast Asia rainforests are being cleared for oil palm plantations. The high water consumption during the cultivation of land plants for the production of biofuels is also viewed critically. Moreover, the current production capacity and area available for this purpose cannot meet the demand for renewable resources for biofuels”. Therefore, despite a century of commercial algal research encompassing many aspects of the value chain (e.g., biology, open pond, closed bio-reactors, bioreactor design, extraction), to date a commercially viable solution has not been reduced to practice that can address the publicly agreed challenges and can meet the market need for large volumes of high quality biomass.